This invention relates to a convergence adjustment for a color TV kinescope display arrangement suited for adjustment on a multipurpose yoke adjustment machine.
Color television kinescopes or picture tubes create images having portions of different colors by causing electrons to impinge upon or illuminate phosphors having different emissions. Normally, phosphors having red, green and blue light emission are used, grouped into myraid trios or triads of phosphor areas, with each triad containing one phosphor area of each of the three colors.
In the kinescope, the phosphors of each of the three colors are illuminated by an electron beam which is intended to impinge upon phosphors of only one color. Each electron beam has a relatively large cross-section compared with a phosphor triad, and each beam illuminates several triads. The three electron beams are generated by three electron guns located in a neck portion of the kinescope opposite the viewing screen formed by the phosphors. The electron guns are oriented so that the beams as generated leave the guns in parallel or somewhat converging paths directed towards the viewing screen. In order to allow the display of a gamut of colors, the phosphor array in a given area must be illuminated by the three electron beams with an intensity dependent upon the color to be displayed. The three electron beams leaving the electron guns in separate parallel paths will if uncorrected illuminate the viewing screen in three different locations, forming separated dots of different colors. In order to enable a single illuminated area to display a color gamut, the electron beams are caused to converge at or near the viewing screen. At the center of the screen, this may be accomplished by the use of a permanent magnet assembly mounted in the neck region of the kinescope for producing a static magnetic field which causes the three beams to converge or register at the center of the viewing screen. This adjustment is known as "static convergence."
With the three electron beams illuminating the same area of the viewing screen, some means must be provided for color separation. This is accomplished by the shadow mask. The shadow mask is a conductive screen or grill having large numbers of perforations through which portions of the electron beams may pass. Each perforation is in a fixed position relative to each triad of color phosphor areas. Portions of the electron beams pass through one or more of the perforations and the portions of each beam begin to diverge and separate from the portions of the other beams as they approach the viewing screen. At the viewing screen the portions are separated and fall upon the appropriate phosphor color based upon the direction of electron beam incidence. That is, each electron beam approaches a given group of perforations from a slightly different direction and the beams are split into a number of smaller beam portions before falling upon the appropriate individual color phosphor areas. The method depends upon a high order of accuracy in the placement of the phosphor triads relative to the perforations and the apparent source of the electron beams. In order to insure that the apparent source of the electron beam is correct, a "purity" adjustment is made by which each beam is caused to illuminate only a particular one of the phosphor areas of each triad.
In order to form a two-dimensional image, the lighted dot on the viewing screen caused by the three statically converged electron beams must be moved both horizontally and vertically over the viewing screen to form a lighted raster area. This is accomplished by means of magnetic fields produced by a deflection yoke mounted upon the neck of the kinescope. The deflection yoke commonly deflects the electron beam with substantially independent horizontal and vertical deflection systems. Horizontal deflection of the electron beam is provided by pairs of conductor arrays of the yoke which produce a magnetic field having vertically extending field lines. The amplitude of the magnetic field is varied with time at a relatively high rate. Vertical deflection of the electron beams is accomplished by pairs of conductor arrays producing a horizontally extending magnetic field which varies with time at a relatively low rate.
A permeable magnetic core is associated with the yoke conductors. The conductors are formed into continuous windings or coils by return conductors which may enclose the core within the coil to form a toroidal deflection winding, or which form a saddle coil winding if the coil does not enclose the core.
The viewing screen is relatively flat. The electron beam, which traverses a given distance from the point or center of deflection to the center of the viewing screen, will traverse a greater distance when deflected towards the edge of the viewing screen. From geometrical considerations, it may be expected that the electron beams will converge at a point on the surface of a sphere centered at the point of deflection. This alone would result in a separation of the landing points of the three electron beams away from the center of the screen. In addition, unavoidable longitudinal components of the deflecting magnetic field cause the electron beams to be more strongly converged whereby the surface at which the beams converge is further distorted. These effects combine to cause the light spots generated by the three beams at points away from the center of the viewing screen to be separated, even though each of the beam illuminates only its appropriate color phosphor. This is known as misconvergence, and results in color fringes about the displayed images. A certain amount of misconvergence is tolerable, but complete separation of the three illuminated spots is generally not. Misconvergence may be measured as a separation of the ideally superimposed red, green and blue lines of a crosshatch pattern of lines appearing on the raster as an appropriate test signal is applied to the receiver.
Formerly, kinescopes had the electron guns in a triangular or delta configuration. Convergence of the electron beams to form a coalesced light spot at points away from the center of the viewing screen was accomplished in delta-gun systems by dynamic convergence arrangements including additional convergence coils mounted about the neck of the kinescope and driven at the deflection rates by dynamic convergence circuits, as described in U.S. Pat. No. 3,942,067 issued Mar. 2, 1976 to Cawood.
As described in U.S. Pat. No. 3,789,258 issued Jan. 29, 1974 to Barbin, and in U.S. Pat. No. 3,800,176 issued Mar. 26, 1974 to Gross and Barkow, current television display arrangements utilize an in-line electron gun assembly together with a self-converging deflection yoke arrangement including deflection windings for producing negative horizontal isotropic astigmatism and positive vertical isotropic astigmatism for balancing the convergence conditions of the beams on the deflection axes and in the corners such that the beams are substantially converged at all points on the raster. This eliminates the need for dynamic convergence coils and circuits. With the increased deflection angles necessitated by commercially desirable short kinescopes, the deflection yoke is required to correct for pincushion and other raster distortions as well as provide satisfactory self-convergence. The magnetic field nonuniformity providing the isotropic astigmatism necessary for self-convergence makes the convergence dependent upon the position of the longitudinal axis of the yoke relative to the longitudinal axis of the kinescope. This sensitivity together with normal manufacturing tolerances makes it necessary to adjust the yoke transversely relative to the kinescope to achieve the best compromise convergence, but may affect the raster distortion. If a position is selected for the yoke in which the raster distortion is satisfactory, there may be a residual convergence error. It is known that placing a permeable tab adjacent the yoke can correct the residual convergence error, but finding the correct side of the kinescope on which to apply the tab, locating the proper position and affixing the tab to the yoke with glue is time-consuming, because the alignment operator is normally in front of the kinescope while performing other alignments, and must be behind the kinescope to add the tabs. It is desirable to have an arrangement by which an alignment operator may conveniently correct residual convergence error.
A convergence correction arrangement for deflection yoke adapted to be disposed about and coaxial with an in-line beam kinescope is described in United States Patent Application Ser. No. 951,001, filed Oct. 13, 1978 now abandoned, in the name of Barkow, et al. This convergence correction arrangement includes first and second magnetic field influencers located on opposite sides of the axis of the kinescope-deflection yoke system. These influencers or tabs are mounted on a plastic slider adapted for vertical movement, thereby providing differential adjustment of the tabs relative to the kinescope.
Kinescope-deflection yoke arrangements are currently aligned with the aid of a yoke adjustment machine (YAM). The yoke adjustment machine is arranged so that the operator can mount a kinescope and yoke from the front of the machine, and remotely control the engagement of the tube socket, the rotational position of each of three sets of magnets associated with the static convergence apparatus, the position of each of four convergence magnets and the x,y,z and rotational position of the yoke relative to the tube. The mechanisms by which these remote connections and adjustments are performed must be nonmagnetic in order to avoid perturbing the adjustment procedure. Consequently, the mechanism is an intricate arrangement of remote-control drive chains, gears and rods actuated by remote motors. The aforementioned convergence correction arrangement as described by Barkow, et al., while satisfactory for a less intricate YAM, has the disadvantage that at the extremes of the positioning of the magnetic tabs the sliding support upon which the tabs are mounted projects beyond the edge of the main portion of the yoke body. Thus, the slider of the Barkow, et al., arrangement may interfere with a YAM mechanism in any but its center position. It is desirable to have a convergence correction arrangement by which magnetic tabs at the rear of the yoke may be moved vertically with a differential motion by a mechanism which does not project beyond the rear of the yoke or farther than the extreme position taken by the tab.